1. Field of the Invention
This invention relates to an image forming method such as an electrophotographic method or an electrostatic recording method for use in an image forming apparatus such as a copying apparatus or a printer.
2. Related Background Art
a) Contact Charging
In an image forming apparatus of the electrophotographic type or the electrostatic recording type or the like, a corona charger has heretofore been generally used as a charging means for charging an image bearing member such as an electrophotographic photosensitive body or an electrostatic recording dielectric body or other member to be charged to a predetermined polarity and potential.
This is such that a corona charger is disposed in non-contact and opposed relationship with an image bearing member (hereinafter referred to as the photosensitive body) and the surface of the photosensitive body is exposed to corona emitted from the corona charger to thereby charge the surface of the photosensitive body to a predetermined polarity and potential.
In recent years, due to advantages such as low ozone, low electric power, etc., as compared with a case using the corona charger of the above-described non-contact type, there has been put into practice a charging device of the contact type in which as previously described, a charging member (contact charging member) having a voltage (charging bias) applied thereto is made to abut against the photosensitive body as a member to be charged to thereby charge the surface of the photosensitive body to a predetermined polarity and potential.
Particularly, a device of a roller charging type using an electrically conductive roller (charging roller) as the contact charging member is preferably used from the viewpoint of the stability of charging.
Also, a magnetic brush charging member (a charging magnetic brush, hereinafter referred to as the magnetic brush charger) provided with a magnetic brush portion having magnetic particles magnetically restrained on a bearing member is used as the contact charging member, and a device of the magnetic brush charging type in which the magnetic brush portion of the magnetic brush charger is brought into contact with the photosensitive body is also preferably used from the viewpoint of the stability of charging contact.
The magnetic brush charger is formed with a magnetic brush portion by electrically conductive magnetic particles being directly magnetically restrained on a magnet or on a sleeve containing a magnet therein, the magnetic brush portion is stopped or rotated and brought into contact with the photosensitive body, and a voltage is applied thereto to thereby start the charging of the photosensitive body.
Also, a member provided with electrically conductive fibers formed into the shape of a brush (a fur brush charging member or a charging fur brush) or an electrically conductive rubber blade (a charging blade) comprising electrically conductive rubber formed into the shape of a blade is preferably used as the contact charging member.
Two kinds of charging mechanisms of the corona charging system and the charge injection charging (direct charging) system are each present in the charging mechanism (the mechanism of charging or the principle of charging) of contact charging, and the characteristic of each-of them appears depending on which is dominant.
The corona charging system is a system whereby the surface of the photosensitive body is charged with a discharge product by a corona discharge phenomenon occurring in the minute gap between the contact charging member and the photosensitive body. Corona charging has a constant discharge threshold value in the contact charging member and the photosensitive body and therefore, it is necessary to apply a voltage greater than charging potential to the contact charging member. Also, a discharge product is produced although markedly small in the quantity of production as compared with a corona charger.
The charge injection charging system is a system in which charges are directly injected from the contact charging member to the photosensitive body, whereby the surface of the photosensitive body is charged. More particularly, it is such that the contact charging member of medium resistance contacts with the surface of the photosensitive body and effects the direct injection of charges into the surface of the photosensitive body without the intermediary of a discharge phenomenon, i.e., basically without the use of discharge. Consequently, even if the applied voltage to the contact charging member is an applied voltage of a discharge threshold value or less, the photosensitive body can be charged to potential corresponding to the applied voltage. This charge injection charging system is not accompanied by the creation of ions.
However, due to injection charging (hereinafter referred to as the injection charging), the contacting property of the contact charging member with respect to the photosensitive body becomes greatly effective for the charging property. So, the contact charging member need adopt a construction which is constructed more densely and which has more speed difference from the photosensitive body and contacts with the photosensitive body with higher frequency, and in this point, as the contact charging member, particularly the magnetic brush charger can effect stable charging.
The injection charging by the magnetic brush charger can be regarded as being equivalent to a series circuit of a resistor and a capacitor. In an ideal charging process, a capacitor is charged for a time during which a certain point on the surface of the photosensitive body is in contact with the magnetic brush (the charging nip.times.the peripheral velocity of the photosensitive body) and the surface potential of the photosensitive body assumes substantially the same value as the applied voltage.
There is a method of applying a voltage to an electrically conductive contact charging member and injecting charges to a trap level on the surface of a photosensitive body to thereby effect the contact charging of the photosensitive body. Also, when as the photosensitive body, use is made of one having a surface layer (charge injecting layer) with electrically conductive particulates dispersed on an ordinary organic photosensitive body, or an amorphous silicon photosensitive body or the like, it is possible to obtain on the surface of a member to be charged charging potential substantially equal to a DC component in the bias applied to the contact charging member (Japanese Patent Application Laid-Open No. 6-3921).
There is also a known method of applying a voltage to an electrically conductive contact charging member, and injecting charges of the same polarity as the potential of a photosensitive body into the photosensitive body having, on the surface thereof, a charge injecting layer with electrically conductive powder (SnO.sub.2 or the like) which is a trap level dispersed thereon to thereby effect contact charging.
The injection charging system is not only small in environment dependency but does not use discharge and therefore, the applied voltage to the contact charging member is sufficient if it is of the same degree as the potential of the photosensitive body, and also has the merit that ozone is not produced, and completely ozoneless and low electric power consumption type charging becomes possible.
b) Cleanerless Process (Toner Recycle Process)
Also, in recent years, downsizing has progressed in image forming apparatuses, but the general downsizing of an image forming apparatus has been limited if means and instruments for image forming processes such as charging, exposure, development, transfer, fixing and cleaning are simply made compact.
Also, any untransferred toner (residual developer) on the photosensitive body after transfer is collected by cleaning means (a cleaner) and becomes waste toner, but it is preferable from the viewpoint of the protection of environment that this waste toner not be discharged.
So, a "cleanerless process" image forming apparatus of a construction in which a cleaner is removed and any untransferred toner on a photosensitive body is removed from the photosensitive body by developing means in "cleaning simultaneous with development" and is collected and reused in the developing means has made its appearance. The cleaning simultaneous with development is a method of collecting some toner residual on the photosensitive body after transfer by a fog removing bias (a fog removing potential difference Vback which is the potential difference between a DC voltage applied to the developing means and the surface potential of the photosensitive body) during the development after the next step. According to this method, the untransferred toner is collected into the developing means and is used after the next step and therefore, waste toner can be done away with and the cumbersomeness of maintenance can be reduced. Also, being cleanerless has a great advantage in terms of space and the image forming apparatus can be made greatly compact.
Also, when the charging device for the photosensitive body is a contact charging device, the untransferred toner is once collected by a contact charging member which is in contact with the photosensitive body, and it is again discharged onto the photosensitive body and is collected by the developing device.
When the injection charging system and reversal developing are used together, a voltage of the polarity opposite to the charging potential is applied to the photosensitive body when a toner image on the photosensitive body is transferred to a transfer material such as paper, and as the result, charges of the opposite polarity are injected into the charge injecting layer on the surface of the photosensitive body.
Almost all of these charges of the opposite polarity are negated by charges of the same polarity injected by the charging device, but when the voltage of the transfer bias is high as compared with the voltage of the injection charging, some of the opposite charges remain in the charge injecting layer and reduce the potential of the photosensitive body. Particularly, the higher is the electrical resistance of the charge injecting layer and the greater is the layer thickness of the charge injecting layer and the lower is the charging capability of injection charging, the deeper portion reach the charges of the opposite polarity injected by transfer, whereas the charges of the same polarity of charging only reach the shallow portion of the charge injecting layer and therefore, the remains of charges of the opposite polarity become much.
A variation in the potential of the photosensitive body by image formation in such a case will now be described with reference to FIGS. 9A, 9B and 9C. In this system, the polarity of the photosensitive body 1 and toner t is the negative polarity. Consequently, the transfer bias is of the positive polarity.
FIG. 9A: this shows the state of the photosensitive body 1 after charging, exposure and development. Only the central portion is exposed, and the negative charges of the charge injecting layer have disappeared. The toner t is present only in that area (reversal developing).
FIG. 9B: the toner t shifts to the transfer material P by a transfer bias. At this time, an electric current flows from a transfer charger 5 to the photosensitive body 1, but since the area in which the toner is present is high in electrical resistance, the electric current flows more into the region around it where the toner is absent. As a result, more of positive charges are injected into the charge injecting layer of the area in which the toner is absent than into the area in which the toner is present.
FIG. 9C: by the injection of negative charges by the contact charging member 2A of a charging device (injection charger), negative charges are injected into the charge injecting layer of the photosensitive body 1, but by the difference in the amount of positive charges which were present in advance, the amount of negative charges in the charge injecting layer in the area wherein the toner t was absent becomes smaller than that in the area wherein the toner t was present. As a result, a difference occurs to the potential of the photosensitive body, and it appears as a density difference in the next image. That is, there occurs a so-called "negative ghost" in which the density of the area in which the toner is present is low.
This negative ghost does not disappear even in the resetting of the potential of the photosensitive body by pre-exposure effected before charging. This is because the work of the pre-exposure is the erasing of the charges of the same polarity as the potential.
Also, in the injection charging system and the cleanerless process, it is difficult to erase the above-mentioned negative ghost even by a pre-charger (to which a voltage of the polarity opposite to that of the photosensitive body is applied) used to charge the untransferred toner to the polarity opposite to that of the charging potential or remove the charges before injection charging, and efficiently effect the collection of the toner by injection charging. This is because the absolute value of the bias applied for the charging or the removal of the charges of the toner is at the same level as that of the charge injection charging bias and therefore it is difficult to inject opposite charges into the depth of the charge injecting layer as in the case of a transfer charger. The application of the same voltage as that during transfer results in the injection of a great deal of opposite charges into the charge injecting layer. When the charging capability of injection charging has lowered, charging cannot be done to desired potential and as the result, fog may be produced in the developing portion. Conversely, when in pre-charging, a voltage of the same magnitude as the transfer voltage and of the same polarity as the potential of the photosensitive body is applied to remove the charges of the opposite polarity in the charge injecting layer in advance, the untransferred toner is charged to the same polarity which is difficult to collect by the injection charger. Therefore, it is apparent that there occurs a positive ghost in which the toner-present area appears thickly in the image of the next round.